A plug-in hybrid electric vehicle (PHEV) and a hybrid electric vehicle (HEV) each have an engine and at least one electric motor, wherein the energy source for the engine is fuel and the energy source for the motor is an electric battery. The PHEV battery has a larger capacity and is primarily rechargeable from an external electric grid. The battery may be charged by the engine using fuel stored in the vehicle.
PHEVs and HEVs buffer fuel energy and recover kinematic energy in electric form to improve the overall operating efficiency. For HEVs, fuel is the principal energy source. For PHEVs, there is an additional source of energy—the amount of electric energy stored in the battery from the grid after each battery charge event. The larger and re-chargeable PHEV battery enables higher capability of fuel energy buffering and kinematic energy recovering and a plug-in recharging ability to thereby improve the fuel economy in both a fuel/electric blended driving mode and an electric driving mode.
HEVs are typically operated to maintain a battery state of charge (SOC) around a constant charge level. In contrast, PHEVs use as much pre-saved battery energy as possible before the next battery charge event; i.e. the relatively low cost grid supplied electric energy is expected to be fully used for propulsion and other vehicle functions after each charge. After the battery SOC decreases to a predefined low level during a given driving event, the PHEV resumes operation as a HEV in a so-called charge sustaining (CS) mode.
To this end, two basic PHEV operating modes include a charge depleting (CD) mode and the charge sustaining (CS) mode. During a first travel distance after a charge, the fully/partially charged PHEV is driven in CD mode, where primarily the battery is used to propel the vehicle, gradually depleting the battery SOC. Once the battery SOC decreases to the low level, the vehicle switches to CS mode, where the battery SOC is kept within a certain range around the low level and the vehicle is mainly powered by fuel energy as is done in a HEV.
The base CD range is the distance a fully charged PHEV can travel in CD mode before the energy utilization pattern switches to the CS mode. By primarily using the battery to propel the vehicle, the fuel consumption is minimized (blended CD mode). The vehicle may even operate with no fuel cost (all-electric CD mode) when the trip distance is less than the base CD range (e.g., ˜30-60 miles). The battery, which will be depleted to a SOC less than a maximum SOC but greater than the low level, can be recharged using the grid during driving off-time.
Three general classifications for HEV power-trains include: series hybrid electric, parallel hybrid electric, and series-parallel hybrid electric. The series-parallel hybrid electric classification includes the so-called power-split hybrid electric power-train.
In the case of a series HEV, the engine drives a generator, which converts mechanical engine power to electrical power. A portion of the electrical power is used to drive the motor, which converts electrical power back to mechanical power to drive the wheels. The power not needed by the motor is used to charge the battery.
In the case of parallel and series-parallel HEVs, mechanical engine power can be delivered to the wheels, and electric battery power can be delivered to the motor, which converts the electric power to mechanical power to drive the wheels. Power flow from the engine to a generator occurs when the battery is being charged. Transmission gearing forms parallel power flow paths to the wheels.
A misconception about parallel and series-parallel HEVs is that electric-only propulsion improves fuel economy because no fuel is used when the engine is off. However, this is not the case because losses incurred by the motor and the battery during discharging and subsequent battery charging can degrade fuel economy as electric power is converted to mechanical power and mechanical power is converted to electric power. Thus, in parallel and series-parallel HEVs that use an engine and a motor powered by a battery, the guiding energy management strategy principle is to provide as much propulsion as possible with the engine, while selectively using the electrical system to increase the average efficiency of the engine.
A parallel or series-parallel HEV that relies upon an electric grid for charging the battery (i.e., PHEV) changes the assumption that electric-only propulsion is fuel inefficient. This is because as there is an externally available energy source, the best fuel economy is achieved by using it, rather than using the on-board fuel, to propel the vehicle. Therefore, energy management strategies are desired to realize the fuel economy improvements that can be gained by using PHEVs.